Ultrasonic occupancy sensors are used to detect the presence of moving objects (e.g., a person) entering an area of interest (e.g., a room). When such a moving object is detected, the ultrasonic occupancy sensors perform a function (e.g., turn on lights in the room, adjust a thermostat). The ultrasonic occupancy sensor radiates (transmits) high frequency sound waves that are undetectable to the human ear. These sound waves bounce off surfaces, including people. Motion is detected via shifts in frequency that are detected as “Doppler shift” when receiving the reflected sound waves and comparing the frequency thereof to the transmitted sound wave frequency in a frequency mixer and then through a low pass filter.
The relative acoustic strength of the high frequency sound waves is affected by many factors including cubic footage of desired coverage, partitions, drapes, carpeting, furniture, potential reflection patterns, and the efficiency of the transducer converting electrical energy into acoustic energy. If adjustments in detection sensitivity are required, present technology ultrasonic occupancy sensors use either a potentiometer (manual) or algorithms in a microcontroller (automatic) to adjust the amplitude of the received signal. Adjusting the amplitude of the received signal is critical to avoid saturation (overload) of the receiver circuitry and to accommodate various noise sources such as heavy airflow from a supply register in the ceiling, floor, and/or wall. Adjusting the detection sensitivity automatically can be referred to as automatic gain control (AGC). Adjustments to the signal can be performed by a preamplifier, also called a preamp.
At times, multiple receivers are used in a single ultrasonic occupancy sensor. By using multiple receivers, the ultrasonic occupancy sensor can cover a larger area compared with an ultrasonic occupancy sensor that uses a single receiver.